Scale effect on rock fissuration porosity

Scale effect on rock fissuration porosity is analyzed by means of structural conceptual schematic diagram designed for a reservoir of cubical blocks separated by clefts with constant openings. Two cases are considered: (1) either the blocks are compact (simple porosity due to clefts), or (2) the blocks are affected by fissuration porosity (in which case the system has double fissural porosity). This hexahedral schematization is consistent with what is often noted in tectonic fissuration. Porosities are calculated for increasing volumes whether they be spherical or cubic, and these porosities are expressed in relation to the average effective porosity of the aggregate. If we refer to the different sizes of the interfissural distances normally observed, we note that the representative porosities can probably be reached only for volumes in excess of 10⁶ m³, even in the best circumstances. (This is the Representative Elementary Volume). For such volumes, an experimental approach seems difficult. Moreover, in natural media, volumes of such magnitude must embody various fields of heterogeneity which adversely affect the significance of apparent porosities. This is the case as shown by a variation within the same rock in the fissuration density. We conclude, therefore, that research into this type of porosity should be carried out by statistical methods, because we are dealing with a parameter that can be analyzed as the function of a regionalized variable and expressed as a term of probability.